Journal of Sedimentary Research, 2019, v. 89, 1096–1108
MICROBIAL STRUCTURES AND DINOSAUR TRACKWAYS FROM A CRETACEOUS COASTAL ENVIRONMENT (DAKOTA GROUP, COLORADO, U.S.A.)
NORA NOFFKE,1 JAMES HAGADORN, 2 AND SAM BARTLETT 3
1 Department of Ocean, Earth & Atmospheric Sciences, Old Dominion University, Norfolk, Virginia 23529, U.S.A.
2 Department of Earth Sciences, Denver Museum of Nature & Science, Denver, Colorado 80205, U.S.A.
3 Friends of Dinosaur Ridge, 16831 West Alameda Parkway, Morrison, Colorado 80465, U.S.A. e-mail: nnoffke@odu
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Microbially induced sedimentary structures may help preserve unique glimpses of ancient shoreline habitats, but are little known from Mesozoic epicontinental settings. To help fill this knowledge gap, we describe a diverse suite of microbial structures from the Upper Cretaceous ‘‘J’’ Sandstone (South Platte Formation, Dakota Group) that are spectacularly exposed at Dinosaur Ridge in Morrison, Colorado, USA. Structures include ‘‘tattered’’ bed surfaces and ferruginous sand chips in supratidal flat facies. A large over-flip structure is preserved in a channel locally known as Crocodile Creek. In upper-intertidal facies, multidirectional ripple marks occur. Perhaps the most well-known microbial structures are exposed on extensive bedding surfaces known as ‘‘Slimy Beach,’’ where lower supratidal-flat facies are dominated by decimeter-scale erosional remnants and pockets. Morphologies and superposition of the structures allows identification of three generations of erosional pockets. Generation A of these erosional pockets exhibit size similarities to ornithomimid, sauropod, and ornithopod dinosaur tracks from adjacent bedding planes, raising the question of whether initial disturbance of the mat-bound surface could have been from track making. Generation B erosional pockets are older and record continuous erosion of the initial pockets until they were eventually overgrown and sealed by microbial mats. Generation C pockets are the oldest ones, exposing wide areas of barren sediment that could not be overgrown by microbial mats anymore. In concert, the microbial structures point to seasonally variable meteorological conditions along the coastline of the Western Interior Seaway and indicate that the ‘‘Slimy Beach’’ bedding plane represents a multi-year record of dinosaur locomotion.